Wireless subscriber communication units, for example those operating in a cellular telephone system such as the Global System for Mobile communications (GSM) or operating on a private mobile radio system such as a TErrestrial Trunked RAdio (TETRA) communication system, use oscillator circuits to generate the radio frequency signals.
An oscillator circuit has two primary functions:                (i) An active device, such as a transistor, to produce power gain; and        (ii) A feedback network, routeing a sufficient amount of the active device's output signal to an input of the active device, to sustain oscillations. In effect, the component values of the feedback network determine the operating frequency. The circuit oscillates when the product of the transistor gain times the network losses (including any mismatches) substantially equals ‘1’. In practice, the product is set to a value slightly higher than ‘1’ so that oscillations can start and build up to a high level.        
A common oscillator implementation is known as a voltage controlled oscillator (VCO) circuit, where an input tuning voltage is applied to an oscillator circuit and the tuning voltage adjusted to set the frequency at which the circuit oscillates. The frequency range of the VCO circuit is limited to circuit component parameters and the tuning voltage used.
Note that a narrow band VCO circuit is typically defined as:(Fmax−Fmin)/Faverage<10% of Fc.
That is, a narrowband VCO is typically considered to have an acceptable frequency range of less than ten percent of its nominal oscillating frequency.
However, the widespread adoption of wireless technologies has created a need to allocate frequency spectrum in separate frequency bands to accommodate all potential users of that communication technology. For example, GSM spectrum has been allocated in both the 800 MHz frequency band as well as the 900 MHz frequency band. In addition, TETRA has been allocated frequency spectrum in the 380–400 MHz frequency band, as well as the 410–430 MHz frequency band.
Thus, to avoid manufacturing different wireless subscriber communication units (terminals) for different frequency bands, wireless subscriber unit manufacturers need to develop wireless subscriber communication units capable of operating over a wider frequency range. In this manner, a single wireless subscriber communication unit is then able to encompass all potential frequencies supported by the respective communication standard.
One option to support a wide frequency range within a single wireless subscriber communication unit is to design two separate VCO circuits.
An alternative approach would be to provide a wideband voltage controlled oscillator (VCO). Developing a single wideband VCO to operate across the desired frequency range saves cost and space, when compared to the alternative solution of designing two separate VCO circuits.
However, the implementation of a ‘wideband’ VCO is more complex. Furthermore, the inventors of the present invention have recognised that a significant disadvantage with such wideband VCO designs is that VCO phase noise degrades over the frequency band. This problem is exacerbated as the VCO bandwidth increases. Increasing the VCO tuning bandwidth, particularly over say 10% of its centre frequency, has noticeably increased this effect in a traditional Colpitts VCO configuration. This effect is noticeable irrespective of the VCO component values used.
In the known Colpitts VCO design, for the sole purpose of tuning the VCO frequency is typically shifted using a single variable capacitor. Occasionally, two variable capacitors may be used, again for the sole purpose of tuning the VCO frequency. It can be shown that increasing a VCO frequency range will increase the amount of capacitance change in the VCO using the variable capacitor. The increased capacitance change will increase the variable capacitance losses as well.
Generally, any attempt to improve a loaded-Q performance at high tuning voltages results in a lower gain margin at low tuning voltages. This, in turn, leads to oscillation start up problems, particularly in a traditional Colpitts VCO configuration.
Thus, there exists a need to provide a wireless communication unit that can operate over a wide frequency range. In particular, there exists a need for an improved VCO design to maintain low phase noise across the wide frequency range.